Method of producing chromium and niobium



United States atent Peusacola,',Fla.,.assignors .to National Research Cor- .poration, a corporation of Massachusetts .No Drawing. Application May20, 1957 SerialNo. 660,029 9 .Claims. (Cl. 75.-84.5) 'I-.his.invention relates to the production of chromium and niobium and more particularly to the production of high purity ;hr.omium and niobium by a process wherein a halide of chromium .or niobium is dissolved in a fusedtsalt. and is reduced to chromium orvniobium metal by the addition of .a molten reducing agent. This application-is ,in .part a continuation of our copending application Serial No. 373,512, filed August 8, 1953.

.:A principal object :of'the invention is to .providean improvedzprocess forpro'ducing high yields ofhighpurity chromium ;or niobium;by the reductioncfa chromium or niobium chloride dissolved :in a fusedsalt.

:Anothenobject of'g'the invention is to providea-process of :the above type which is cheaper to operate than heretofore existing processes producing chromium or niobium metal of equivalent purity.

:Otherobjects of thesinvention will in part be obvious and-will in part appear hereinafter.

The .inventionaccordingly comprises the process in volving theseveral steps .and the relation and the order of one or more of such steps with respect to each of the others which are exemplified in the following detailed disclosure, and the scope of the application of which Will-be indicated in the claims.

For a fuller understanding of the nature andobjects of:.the invention, reference should be had :to the following detailed description.

:In the present invention, advantage is taken of the ability of .the alkali and alkaline earth metals .(including magnesium) toreduce chromium-or niobium chloridesito-the corresponding metal. For simplicity of illustration the invention will be initially described in connection with its .use in the production of chromium by the use :of chromium trichloride as a starting material. vfSince the reductio'n'to metal is preferably accomplished at an elevated temperature of about 800 C. "and since chromiumtrichloride is extremely corrosive to metals at these temperatures, certain problems are encountered if .high purity is to 'be maintained. The corrosive nature of "the chromium 'trichloride is such that ceramic or ceramic-lined chambers and .crucibles'are' utilized for "the reduction of the chromium 'trichloride to the dichloride. However, since all ceramics are attacked to some degree by strong reducing agents, .the subsequent reduction of the chromium dichloride to chromium metal is carried out in a metal crucible. The chromiumdichloride is more stable than the trichloride and the metal crucible is not substantially attacked.

We :have discovered that, in such a system, crystal growth is strongly promoted if the reducing agent be supplied to the chromium dichloride fused salt solution via an interposed shielding layer of fused salt which is substantially free of the chromium dichloride and which is preferably localized 'by, and at least in part incorporated in, a solid, porous diaphragm-which is in direct contact with the solution. We have also foundthat such a shielding layer and porous diaphragm can be established and maintained by proper control of the conditions of the bath and-the manner of supplying the reducing agent thereto.

,For example, in one preferred embodiment of .the invention, -asolution of chromium trichloride in fused so- 2,848,320 Patented Aug. 19, 1958 dium chloride is provided in a reactor and-liquid sodium is fed to the surface of the molten salt solution in an amount slightly in excess of that sufiicient to reduce substantially all the chromium trichloride to dichloride. The dichloride fused salt solution is then transferred .to a metal reactor and liquid sodium is fed :to the surface of the molten salt solution as a thin'film ,distributedcsubstantially uniformly over the surface while the fused salt solution remains relatively quiescent. Under these conditions, a crust of sintered, fine chromiumparticles forms rapidly at the surface of the salt batheXtending over said surface and adhering tothe walls of the reactor. This crust is porous and incorporates alayer offusedvsalt which is substantially free of chromium dichloride :at'

by a layer of molten salt which is substantiallyqfree -;of

chromium dichloride. Consequently, 'further feed of sodium is to the surface of this chromium dichloridefree layer of salt above the chromium crust. When the crust and. shielding layer havebeenestablishegl, chromium crystals begin to grow, growingout of the .under surface of the crust and outwardly from the walls of the reactor.

The feed of sodium is preferably continued .until sufficient sodium has been fed to reduce substantially all of the dissolved chromiumdichloride to chromium metal. After the-sodium feed is stopped,- the reactor-is allowed to cool and the solidified contents are leached by-water, thereby removing the salt andleaving substantially ;.p ure chromium metal.

In order to describe more fully preferred-methods of practicing the invention, there is set forth below a nonlimiting example'which is merely illustrative. -lnrthis example, step one involved thepreparationofa CrC l NaCl mixture in a zirconia crucibleby sodium reduction of iCrCl This mixture was then further vreduced by sodium -to metal in a nickel reactor in the second-step. A description of the procedure follows:

Example 1 the top flange into the charge and ;the.assembledunit placed in a furnace. The temperature was brought up slowly. It was allowed to .leveloif at .800" *C. and was held at this temperature forthree hours. An.argonat-.

mosphere was maintained during all ..operations. ,The product was removed, crushed and homogenized,.,and .a sample was submitted for analysis.

to detect any trace of CrCl In the second step, 420 grams of .the CrCl NaC1 I mixture prepared in step one .were placedat the bottom of a 1%" internal diameter by 10 nickelbomb. .A 1.15 diameter nickel disc was supported :above ,the mixture by pins welded in the bomb.- Thena 2" layer of pure NaCl was poured in. The nickel disc separated the charge and the salt barrier. A sodium droppingcrucible v .was supported in the bomb at a point 1 2'"'fabove the The analysis failed a salt barrier. The crucible contained a salt plug and 8.0 grams of sodium (6.82 grams were theoretically needed). Finally, a stainless steel pipe, closed at one end and slightly smaller in outside diameter than the internal diameter of the nickel bomb, was slipped partway into the bomb and welded.

The bomb was then lowered into a hot furnace. When the charge and the salt barrier had melted, the bomb was positioned in the furnace so that the salt plug in the dropping crucible would melt and the sodium was allowed to drop to the top of the molten salt barrier. The temperature at the bottom of the bomb was maintained at 825 C. for 6 hours. The temperature at the top of the salt barrier during the run was 850 C.

At the conclusion of the run, the product was leached in alcohol to remove the excess sodium. This was followed by three water leaches. The chromium metal produced had a purity of 99.9 percent.

While a specific example has been given above, numerous alternative methods may be employed without departing from the scope of the invention. The temperature of the reaction mass may be varied widely from slightly above the melting point of the salt to temperatures on the order of 1000 C. or more. Numerous reducing agents other than sodium may be employed. For example, potassium, calcium, magnesium, lithium and various combinations of these elements may be utilized. From the standpoint of economy, sodium or magnesium is preferred. A ceramic or ceramic-lined vessel may be employed for the first stage reaction and a metal vessel may be used for the second stage reaction. Stabilized zirconia and nickel or a nickel alloy are preferred. All operations must be carried out in an inert atmosphere.

While the invention has been primarily described in connection with the production of chromium, it is equally applicable to the production of niobium. In this case the starting material is a halide of niobium, preferably a lower chloride such as the trichloride which is dissolved in a salt such as sodium chloride. Thus in Example I the chromium dichloride solution used for the second stage can be replaced by a solution of niobium trichloride (or a mixture of the diand trichlorides) in sodium chloride. This solution is then preferably reduced to niobium metal by sodium or other reducing agent. In this case the layer of niobium chloride-free salt is preferably established at the top of the solution of niobium chloride and the sodium is fed to the surface of the niobium chloride-free layer of salt in the same mannor as that employed in reducing the chromium dichloride solution to chromium.

It should be pointed out that the salt mixture in which the reduction is carried out may be formed of numerous halides which may be mixed halides, single halides and halides of materials other than the specific reducing agent or agents employed in the reaction. From the standpoint of simplicity of operation and ease of control, however, it is preferred that the salt be the chloride of the reducing agent. Nevertheless, it is quite feasible to employ binary and ternary mixtures of halides having relatively low melting points.

In addition to the production of pure niobium or chromium, alloys of these metals with other metals can be readily prepared by co-reduction of one of the chlorides of niobium or chromium with a chloride of vanadium, manganese, iron, nickel, cobalt, titanium, thorium, etc. The alloy may be a binary alloy or it may be an alloy containing 3 or 4 constituents. Accordingly, when the expressions chromium and niobium are used in the appended claims, they are intended to include alloys of these metals as well as the pure metals.

Since certain changes may be made in the above process without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A process for preparing chromium metal which comprises the steps of providing a mixture of a molten salt and chromium trichloride in an air-free reduction chamber, maintaining said mixture at a temperature above its melting point, introducing into said mixture a predetermined amount of reducing agent comprising at least one metal selected from the group consisting of the alkali and alkaline earth metals, said reducing agent being sulficient to reduce substantially all the chromium trichloride to chromium dichloride, subsequently reducing said chromium dichloride to chromium metal by feeding the reducing agent to the surface of a layer of salt substantially free of chromium dichloride which is maintained above the dichloride solution, and thereafter separating the chromium metal from the salt.

2. The process of claim 1 wherein the reducing agent comprises sodium and the salt comprises sodium chloride.

3. The process of claim 1 wherein the two stages of the reaction are carried out in separate chambers.

4. The process of claim 1 wherein a chamber comprising ceramic material is used for the first stage reaction.

5. The process of claim 1 wherein a metal crucible is used for the second stage reaction.

6. A process for preparing a substantially pure metal from the group consisting of chromium and niobium which comprises providing on the surface of a fused salt bath containing a dissolved chloride of the metal, a salt layer substantially free of the metal chloride and thereafter feeding to the top of said chloride-free salt layer suflicient reducing agent to reduce substantially all the dissolved metal chloride to said metal, said reducing agent consisting of at least one metal selected from the group consisting of the alkali metals and the alkaline earth metals.

7. The process of claim 6 wherein the metal chloride comprises niobium chloride.

8. In the process for manufacturing chromium wherein a chromium halide is dissolved in a bath of a fused salt and is reduced to chromium crystals by supplying a metallic reducing agent to the bath, the reducing agent comprising a metal selected from the class consisting of the alkali metals and the alkaline earth metals and the fused salt comprising a halide selected from the class consisting of the alkali metal halides and the alkaline earth metals, the improvement which comprises main taining at the surface of the salt bath containing the dissolved chromium halide a zone comprising fused salt which is substantially free of chromium halide, feeding the reducing agent to the surface of the layer of salt which is 1 substantially free of chromium halide, and permitting said reducing agent to pass through the layer of salt for reaction with the chromium halide.

9. In the process for manufacturing niobium wherein a niobium halide is dissolved in a bath of a fused salt and is reduced to niobium crystals by supplying a metallic reducing agent to the bath, the reducing agent comprising a metal selected from the class consisting of the alkali metals and the alkaline earth metals and the fused salt comprising a halide selected from the class consisting of the alkali metal halides and the alkaline earth metal halides, the improvement which comprises maintaining I stantially free of niobium halide, and permitting said reducing agent to pass through the layer of salt for reaction with the niobium halide.

References Cited in the file of this patent UNITED STATES PATENTS Kroll June 25, 1940 Glasser et al Mar. 8, 1955 

6. A PROCESS FOR PREPARING A SUBSTANTIALLY PURE METAL FROM THE GROUP CONSISTING OF CHROMIUM AN NIOBIUM WHICH COMPRISES PROVIDING ON THE SURFACE OF A FUSED SALT BATH CONTAINING A DISSOLVED CHLORIDE OF THE METAL, A SALT LAYER SUBSTANTIALLY FREE OF THE METAL CHLORIDE AND THERE AFTER FEEDING TO THE TOP OF SAID CHLORIDE-FREE SALT LAYER SUFFICIENT REDUCING AGENT TO REDUCE SUBSTANTIALLY ALL THE DISSOLVED METAL CHLORIDE TO SAID METAL, SAID REDUCING AGENT CONSISTING OF AT LEAST ONE METAL SELECTED FROM THE GROUP CONSISTING OF THE ALKALI METALS AND THE ALKALINE EARTH METALS. 